The present invention relates to a new type of sonde to carry out logging measurements in a hole drilled through geological formations.
It is already known to use sondes provided with active elements or transducers (transmitters and receivers) to perform measurements in either cased or uncased boreholes.
The purpose of such measurements is, for example, to determine geological characteristics of the drilled formations, or to ascertain whether metallic tubings positioned in the borehole are properly cemented, etc.
Two sonde types are presently used. In the first type of sonde the transducers are fixed to the sonde body or to pads integral with this body and are in contact with the borehole wall. Embodiments of this first type of sonde are, for example, illustrated in French Patent specification No. 1,121,292 and U.S. Pat. Nos. 2,855,685; 3,356,146 and 3,798,966.
The main drawback of this type of sonde is the wear of the transducers resulting from their friction on the borehole wall during the displacement of the sonde. A further drawback arises, at least in the case of measurements performed in open holes, from the uncertainty as to a proper contact between the transducers and the borehole wall, owing to cross-section variations of the borehole along its height.
This requires an interpretation of the measurements which is always difficult and time-consuming. It should be also noted that the displacement of this type of sonde is sometimes made difficult by the mud "cake" of variable thickness which covers the borehole wall. Moreover this type of sonde provides measurements only over a limited portion of the periphery of the borehole.
A second type of sonde has been designed to prevent the wear of the transducers. In this type of sonde the transducers carried by the sonde body are not in contact with the borehole wall. This is obtained by aligning the sonde body with the borehole axis by using centering means. The irregular shape of the borehole cross-section makes this centering operation difficult. There results therefrom a substantial alteration of the measurements which is further altered by possible variations of some characteristics of the mud filling the borehole, such as, for example, the specific gravity thereof. An interpretation of the performed measurements is therefore also necessary. Modifications of this type of sonde make it also possible to carry out measurements substantially along a generatrix of the borehole, but if the transducers are mounted for rotation about the sonde axis it is possible to perform measurements in all directions. Examples of embodiments of this second type of sonde are shown in by British Pat. No. 1,217,544 and U.S. Pat. Nos. 3,378,097 and 3,614,891.
It is also known in the art of electric logging to use circular electrodes rotatable about axes perpendicular to the sonde axis, these electrodes being preferably provided with teeth capable of performing the mud cake so as to come into direct contact with the geological formations (U.S. Pat. No. 2,307,887).